By this the carnival greeting "Helau" originated from a burlesque ("Fastnachtsspiel") played in Düsseldorf in 1833, the date when carnival beacame officially mentioned there.

Digging the etymology further down to the question why they used "Helau" in the burlesque may come to the point that it simply was a salutation. We have a nice Wikipedia summary on the origin of the English "Hello" (pronounced very similarly):

Hello, with that spelling, was used in publications as early as 1833.
[...]
hello is an alteration of hallo, hollo, which came from Old High German"halâ, holâ, emphatic imperative of halôn, holôn to fetch, used especially in hailing a ferryman." It also connects the development of hello to the influence of an earlier form, holla, whose origin is in the French holà (roughly, 'whoa there!', from French là 'there'). As in addition to hello, hallo and hollo, hullo and (rarely) hillo also exist as variants or related words, the word can be spelt using any of all five vowels.